1. Field of the Invention
This invention relates to a chirp synthesizer and, more specifically, to a high speed digital integrated circuit (IC) which generates the inputs required by a digital to analog (D/A) converter to synthesize a given single frequency waveform or a sweeping frequency or "chirp" signal.
2. Brief Description of the Prior Art
Direct digital synthesis (DDS) of sinusoidal waves is well known and has been successfully demonstrated in the prior art. Such synthesizers offer wide bandwidth, fine frequency resolution, low phase noise and exceptionally fast frequency switching capability and find use in communication and electronic warfare (EW) systems. Linear F.M. (chirp) waveform generators are frequently used in military systems, such as compressive receivers in radar and EW systems. The swept oscillator function is typically performed with a surface acoustic wave (SAW) expander or with discrete digital ECL or GaAs components.
Digital generation of chirp waveforms is based upon the architecture of direct digital synthesizers. In the prior art, such synthesizers have been confined to the generation of low frequency sinusoidal signals in small increments in VHF and UHF systems due to limitations imposed by the speed of the digital circuits available and the inability to integrate the complex functions on a small number of chips. Examples of such prior art are set forth in articles of Michael P. LaMacchia et al., "Flight GaAs Numerically Controlled Oscillator", Technical Digest, 1989 GaAs Symposium, pp 49-51 and J. Browne, "All GaAs Lineup Fuels High-Speed Digital Synthesizer" Microwave & RF, July 1988, pp 129-131. Such chips contained one accumulator and/or one ROM interfacing to a digital to analog converter chip. Recent advances in GaAs IC technology have improved the chip speed and integration potential. However, due to the relative complexity of the frequency synthesizing function and waveforms desired, such complex waveforms have not been directly synthesized.
Digital synthesis of chirp waveforms has been discussed in the prior art in articles by Richard Appis et al., "A Digital Linear FM Generator for Multi-Mode RADAR" Proceeding of the Tri-Services RADAR Symposium, 1988 and Eric Munro et al., "Vector Arbitrary Waveform Synthesis Simplifies Chirp Radar Test", Microwave Systems News, March 1981, pp. 22-25. These articles relate to large systems constructed with existing components.
The art has been unable, to date, to provide a chirp synthesizer in a monolithic circuit which also provides the required specifications available in the prior art.